Part III: Backpropagation mechanics for a Convolutional Neural Network

In part-II of this article, we derived the weight update equation for the backpropagation operation of a simple Convolutional Neural Network (CNN). The input for the CNN considered in part-II is a grayscale image, hence, the input is in the form of a single 4x4 matrix. CNNs used in practice however, use color images where each of the Red, Green and Blue (RGB) color spectrums serve as input. Hence, a CNN with the inputs color coded with their respective spectrums look like:

In this article we expand upon the original CNN where the input is represented by 3, 4x4 matrices each presenting the pixel intensities for the RGB color spectrum. Hence is we represent each of the pixels with the CNN given is represented by:

In this CNN, there are 3, 4x4 input matrices, one 2x2 filter matrix (also known as kernel), a single convolution layer with 1 unit, a single ReLu layer, a single pooling layer (which applies the MaxPool function) and a single fully connected (FC) layer. The elements of the filter matrix are equivalent to the unit weights in a standard NN and will be updated during the backpropagation phase.

Assuming a stride of 2 with no padding, the size of the convolution layer is determined by the following equation:

$$N={(W - F + 2.P)\over S} + 1$$
Here,
N: is the dimension (rows and columns) of the new(convolution) square matrix
W: is the dimension (rows and columns) of the input square matrix
F: is the dimension (rows and columns) of the filter square matrix
P: is the number of pixels(cells) of padding added
S: is the stride, or the number of cells skipped each time the kernel is slided

Putting these values in the equation,
$$\begin{align} N&={(4 - 2 + 2.0)\over 2} + 1\\ &=2 \end{align}$$
Hence the convolution layer will have a matrix of size 2x2.

It is interesting to note that downstream of the convolutional layer, the CNN has the same architecture as the one we used in part-II. Hence, the backpropagation equations will be same, except values of $c_{00},c_{10},c_{10},c_{11}$ will be different, as they now include the values of all the 3 matrices. These values are given below:

$$\begin{align} c_{00}=&(r_{00}.w_{11} + r_{10}.w_{01} + r_{01}.w_{10} + r_{11}.w_{00}) + \\ &(g_{00}.w_{11} + g_{10}.w_{01} + g_{01}.w_{10} + g_{11}.w_{00}) + \\ &(b_{00}.w_{11} + b_{10}.w_{01} + b_{01}.w_{10} + b_{11}.w_{00}) \\ c_{10}=&(r_{20}.w_{11} + r_{30}.w_{01} + r_{21}.w_{10} + r_{31}.w_{00}) + \\ &(g_{20}.w_{11} + g_{30}.w_{01} + g_{21}.w_{10} + g_{31}.w_{00}) + \\ &(b_{20}.w_{11} + b_{30}.w_{01} + b_{21}.w_{10} + b_{31}.w_{00})\\ c_{01}=&(r_{02}.w_{11} + r_{12}.w_{01} + r_{03}.w_{10} + r_{13}.w_{00}) + \\ &(g_{02}.w_{11} + g_{12}.w_{01} + g_{03}.w_{10} + g_{13}.w_{00}) + \\ &(b_{02}.w_{11} + b_{12}.w_{01} + b_{03}.w_{10} + b_{13}.w_{00})\\ c_{11}=&(r_{22}.w_{11} + r_{32}.w_{01} + r_{23}.w_{10} + r_{33}.w_{00}) +\\ &(g_{22}.w_{11} + g_{32}.w_{01} + i_{23}.w_{10} + g_{33}.w_{00}) + \\ &(b_{22}.w_{11} + b_{32}.w_{01} + b_{23}.w_{10} + b_{33}.w_{00})\\ \end{align}$$

Let us now derive the backpropagation.

The Error $E$ is defined as $E = {(t - r_{fc})^2\over 2}$, where t is given(expected) output in the training data.

We need find out change in $E$ in relation to the changes in values of weights $w_{fc}$ and $w_{00}$ through $w_{11}$ given in the filter.

Let us first find out how $E$ changes in relation to $w_{fc}$ or the rate of change of $E$ w.r.t. $w_{fc}$. This is also called the derivative of $E$ w.r.t $w_{fc}$ or $\partial E\over \partial w_{fc}$

$$\begin{align} E &= {(t - r_{fc})^2\over 2}\\ {\partial E\over \partial w_{fc}}&={\partial {(t - r_{fc})^2\over 2}\over \partial w_{fc}}\\ &={1 \over 2}.{\partial {(t - r_{fc})^2}\over \partial w_{fc}} \text{ ... eq.(1.1)}\\ \end{align}$$

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As per the chain rule,:
$${dx \over dy}= {dx \over dz}.{dz \over dy}$$
For a partial derivative:
$${\partial x \over \partial y}={\partial x\over \partial z}.{\partial z\over \partial y}$$
Applying the chain rule to right term of eq.(1.1):
$$\text{Here, } z = {t - r_{fc}},x=z^2, \text{ and } y=w_{fc} \text{ ,therefore, }\\ \begin{align} {1 \over 2}.{\partial {(t - r_{fc})^2}\over \partial w_{fc}}&={1 \over 2}.{\partial x\over \partial y}\\ &={1 \over 2}.{\partial x\over \partial z}.{\partial z\over \partial y}\\ &={1 \over 2}.{\partial z^2\over \partial z}.{\partial z\over \partial y}\\ &={1 \over 2}.{\partial (t - r_{fc})^2\over \partial (t - r_{fc})}.{\partial (t - r_{fc})\over \partial w_{fc}}\\ &={1 \over 2}.{2(t - r_{fc})}.{\partial (t - r_{fc})\over \partial w_{fc}} \text{ ... eq.(1.2)}\\ \end{align}$$
Further,
$$\begin{align} {\partial (t - r_{fc})\over \partial w_{fc}}&={\partial (t - w_{fc}.r_p)\over \partial w_{fc}}\\ &={\partial t\over \partial w_{fc}} - {\partial w_{fc}.r_p\over \partial w_{fc}}\\ &={\partial t\over \partial w_{fc}} - r_p.{\partial w_{fc}\over \partial w_{fc}}\\ &=0 - r_p \text{ (as,} {\partial C \over \partial x}=0 \text { and } {\partial x \over \partial x}=1, \text {where C is a constant)},\\ \end{align}$$
Based on the above, in eq.(1.2),
$$\begin{align} {1 \over 2}.{\partial {(t - r_{fc})^2}\over \partial w_{fc}}&={1 \over 2}.{2(t - r_{fc})}.{\partial (t - r_{fc})\over \partial w_{fc}}\\ &={1 \over 2}.{2(t - r_{fc})}.{(0 - r_p)}\\ &=(t - r_{fc}).(-r_p) \end{align}$$

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Therefore,
$$\begin{align} \partial E\over \partial w_{fc}&={\partial {(t - r_{fc})^2\over 2}\over \partial w_{fc}}\\ &={1 \over 2}.{\partial {(t - r_{fc})^2}\over \partial w_{fc}} \\ &=(t - r_{fc}).(-r_p) \text{ ... eq.(1.3)}\\ \end{align}$$

Further upstream the only variables that control the value of $r_{fc}$ are in the filter F. Hence, we need to find out how E changes in relation to F or ${\partial E \over \partial F}$.

Since the pooling layer applies the MaxPool function, output $r_p$ of pooling layer is given as:
$r_p=max\{u_{00},u_{10},u_{10},u_{11}\}$The ReLu function f(x) is defined as: $f(x)=max(0,x)$
Hence, the values of $u_{00},u_{10},u_{10},u_{11}$ are given as (see CNN article for more details):
$$\begin{align} u_{00}=max(0,&c_{00})\\ =max(0,&(r_{00}.w_{11} + r_{10}.w_{01} + r_{01}.w_{10} + r_{11}.w_{00}) + \\ &(g_{00}.w_{11} + g_{10}.w_{01} + g_{01}.w_{10} + g_{11}.w_{00}) + \\ &(b_{00}.w_{11} + b_{10}.w_{01} + b_{01}.w_{10} + b_{11}.w_{00}))\\ u_{10}=max(0,&c_{10})\\ =max(0,&(r_{20}.w_{11} + r_{30}.w_{01} + r_{21}.w_{10} + r_{31}.w_{00}) + \\ &(g_{20}.w_{11} + g_{30}.w_{01} + g_{21}.w_{10} + g_{31}.w_{00}) + \\ &(b_{20}.w_{11} + b_{30}.w_{01} + b_{21}.w_{10} + b_{31}.w_{00}))\\ u_{01}=max(0,&c_{01})\\ =max(0,&(r_{02}.w_{11} + r_{12}.w_{01} + r_{03}.w_{10} + r_{13}.w_{00}) + \\ &(g_{02}.w_{11} + g_{12}.w_{01} + g_{03}.w_{10} + g_{13}.w_{00}) + \\ &(b_{02}.w_{11} + b_{12}.w_{01} + b_{03}.w_{10} + b_{13}.w_{00}))\\ u_{11}=max(0,&c_{11})\\ =max(0,&(r_{22}.w_{11} + r_{32}.w_{01} + r_{23}.w_{10} + r_{33}.w_{00}) + \\ &(g_{22}.w_{11} + g_{32}.w_{01} + i_{23}.w_{10} + g_{33}.w_{00}) + \\ &(b_{22}.w_{11} + b_{32}.w_{01} + b_{23}.w_{10} + b_{33}.w_{00}))\\ \end{align}$$
Note that for the sake of simplicity, we do not normalize values of $c_{00},c_{10},c_{10},c_{11}$.

When you implement a CNN, the pooling layer has to remember the indices of the convolution layer filter maps that were used as input to the pooling layer. This is the approach that has been used in the Dasmic machine learning library. In this case, let us just assume that $u_{10}$ was the output of the pooling layer, or:
$$u_{10}=max\{u_{00},u_{10},u_{10},u_{11}\}\\ \Rightarrow r_p = u_{10} \text{ ... eq.(1.4)}$$
To compute ${\partial E \over \partial F}$, we will need to find out how E changes in relation to each element of matrix F, or we need to find,${\partial E \over w_{00}},{\partial E \over w_{10}},{\partial E \over w_{01}},{\partial E \over w_{11}}$.

Let us now compute ${\partial E \over \partial w_{00}}$,

Applying the chain rule,
$$\begin{align} {\partial x \over \partial y}&={\partial x\over \partial z}.{\partial z\over \partial y}\\ \text{Here, } z = u_{10}, x&=E, \text{ and } y=w_{00} \text{ ,therefore, }\\ {\partial E \over \partial w_{00}}&={\partial E \over \partial u_{10}}.{\partial u_{10} \over \partial w_{00}} \text{ ... eq.(2.1)}\\ \end{align}$$
Solve for the right term ${\partial u_{10} \over \partial w_{00}}$ in eq.(2.1):
$$\begin{align} {\partial u_{10} \over \partial w_{00}}&={\partial (max(0,c_{10})) \over \partial w_{00}} \text{ ... eq.(2.2)} \end{align}$$
Applying the chain rule to eq.(2.2),
$$\begin{align} {\partial x \over \partial y}&={\partial x\over \partial z}.{\partial z\over \partial y}\\ \text{Here, } z = c_{10}, x&=max(0,c_{10}), \text{ and } y=w_{00} \text{ ,therefore, }\\ {\partial (max(0,c_{10})) \over \partial w_{00}}&={\partial max(0,c_{10})\over \partial c_{10}}.{\partial c_{10}\over \partial w_{00}} \text{ ... eq.(2.3)}\\ \end{align}$$
Now,
$${d {{max(0,x)} \over {dx}}}= \begin{cases} x, & \text {if x > 0}\\ 0, & \text {if x < 0}\\ undetermined, & \text {if x = 0 ... eq.(2.4)} \end{cases}$$ Let us assume that $x \ne 0$. When $x=0$ there are several ways of dealing with it including using a slightly modified ReLu function, but that is outside the scope of this article. Let's now look at the two cases when $x=c_{10}$:

Case 1: $x \lt 0$,

In eq.(2.3),
$$\begin{align} {\partial (max(0,c_{10})) \over \partial w_{00}}&={\partial max(0,c_{10})\over \partial c_{10}}.{\partial c_{10}\over \partial w_{00}}\\ &={\partial 0\over \partial c_{10}}.{\partial c_{10}\over \partial w_{00}} \text { (applying value of } max(0,c_{10}) \text { from eq.(2.4))}\\ &=0.{\partial c_{10}\over \partial w_{00}}\\ &=0\text{ ... eq.(2.5)} \end{align}$$
Putting value from eq.(2.5) in eq.(2.2),
$$\begin{align} {\partial u_{10} \over \partial w_{00}}&={\partial (max(0,c_{10})) \over \partial w_{00}}\\ &=0 \end{align}$$
Putting value of $\partial u_{10} \over \partial w_{00}$ in eq.(2.1),
$$\begin{align} {\partial E \over \partial w_{00}}&={\partial E \over \partial u_{10}}.{\partial u_{10} \over \partial w_{00}}\\ &={\partial E \over \partial u_{10}}.0\\ &=0\\ \end{align}$$
Case 2: $x \gt 0$,
In eq.(2.3),
$$\begin{align} {\partial (max(0,c_{10})) \over \partial w_{00}}&={\partial max(0,c_{10})\over \partial c_{10}}.{\partial c_{10}\over \partial w_{00}}\\ &={\partial c_{10}\over \partial c_{10}}.{\partial c_{10}\over \partial w_{00}} \text { (applying value of } max(0,c_{10})=c_{10} \text { from eq.(2.3))}\\ &=1.{\partial c_{10}\over \partial w_{00}}\\ &={\partial c_{10}\over \partial w_{00}} \text{ ... eq.(2.6)} \end{align}$$
Solving for eq.(2.6),
$$\begin{align} {\partial c_{10}\over \partial w_{00}}=&\partial ((r_{20}.w_{11} + r_{30}.w_{01} + r_{21}.w_{10} + r_{31}.w_{00}) + \\ &(g_{20}.w_{11} + g_{30}.w_{01} + g_{21}.w_{10} + g_{31}.w_{00}) + \\ &{(b_{20}.w_{11} + b_{30}.w_{01} + b_{21}.w_{10} + b_{31}.w_{00}))\over \partial w_{00}} \text { (substituting full value of } c_{10} \text{)}\\ =&{\partial {(r_{20}.w_{11})} \over \partial w_{00}}+ {\partial {(r_{30}.w_{01})} \over \partial w_{00}} + {\partial {(r_{21}.w_{10})} \over \partial w_{00}} + {\partial {(r_{31}.w_{00})} \over \partial w_{00}} +\\ &{\partial {(g_{20}.w_{11})} \over \partial w_{00}}+ {\partial {(g_{30}.w_{01})} \over \partial w_{00}} + {\partial {(g_{21}.w_{10})} \over \partial w_{00}} + {\partial {(g_{31}.w_{00})} \over \partial w_{00}} +\\ &{\partial {(b_{20}.w_{11})} \over \partial w_{00}}+ {\partial {(b_{30}.w_{01})} \over \partial w_{00}} + {\partial {(b_{21}.w_{10})} \over \partial w_{00}} + {\partial {(b_{31}.w_{00})} \over \partial w_{00}} \\ =&{\partial {(r_{20}.w_{11})} \over \partial w_{00}}+ {\partial {(r_{30}.w_{01})} \over \partial w_{00}} + {\partial {(r_{21}.w_{10})} \over \partial w_{00}} + r_{31}.{\partial {w_{00}} \over \partial w_{00}}+\\ &{\partial {(g_{20}.w_{11})} \over \partial w_{00}}+ {\partial {(g_{30}.w_{01})} \over \partial w_{00}} + {\partial {(g_{21}.w_{10})} \over \partial w_{00}} + g_{31}.{\partial {w_{00}} \over \partial w_{00}}+\\ &{\partial {(b_{20}.w_{11})} \over \partial w_{00}}+ {\partial {(b_{30}.w_{01})} \over \partial w_{00}} + {\partial {(b_{21}.w_{10})} \over \partial w_{00}} + b_{31}.{\partial {w_{00}} \over \partial w_{00}} \text{ ... eq.(2.7)}\\ \end{align}$$
Note that the terms $({r_{20}.w_{11}}), ({r_{30}.w_{01}}), ({r_{21}.w_{10}}),({g_{20}.w_{11}}), ({g_{30}.w_{01}}), ({g_{21}.w_{10}}),({b_{20}.w_{11}}), ({b_{30}.w_{01}})$ and $({b_{21}.w_{10}})$ will not change w.r.t $w_{00}$ hence, can be treated as a constant C
As ${\partial C \over \partial x} = 0$, in eq.(2.7),
$$\begin{align} {\partial c_{10} \over \partial w_{00}}=&{\partial {(r_{20}.w_{11})} \over \partial w_{00}}+ {\partial {(r_{30}.w_{01})} \over \partial w_{00}} + {\partial {(r_{21}.w_{10})} \over \partial w_{00}} + r_{31}.{\partial {w_{00}} \over \partial w_{00}}+\\ &{\partial {(g_{20}.w_{11})} \over \partial w_{00}}+ {\partial {(g_{30}.w_{01})} \over \partial w_{00}} + {\partial {(g_{21}.w_{10})} \over \partial w_{00}} + g_{31}.{\partial {w_{00}} \over \partial w_{00}}+\\ &{\partial {(b_{20}.w_{11})} \over \partial w_{00}}+ {\partial {(b_{30}.w_{01})} \over \partial w_{00}} + {\partial {(b_{21}.w_{10})} \over \partial w_{00}} + b_{31}.{\partial {w_{00}} \over \partial w_{00}}\\ =& 0 + 0 + 0 + r_{31}.{\partial {w_{00}} \over \partial w_{00}} + 0 + 0 + 0 + g_{31}.{\partial {w_{00}} \over \partial w_{00}} + 0 + 0 + 0 + b_{31}.{\partial {w_{00}} \over \partial w_{00}}\\ =&0 + 0 + 0 + r_{31} + 0 + 0 + 0 + g_{31} + 0 + 0 + 0 + b_{31} (\text {as, }{\partial x \over \partial x}=0 \text {)}\\ =&r_{31} + g_{31} + b_{31} \text{ ... eq.(2.8)} \end{align}$$
Putting value from eq.(2.8) in eq.(2.2),
$$\begin{align} {\partial u_{10} \over \partial w_{00}}&={\partial (max(0,c_{10})) \over \partial w_{00}}\\ &={\partial c_{10} \over \partial w_{00}} \text { (from eq.(2.6))}\\ &=r_{31} + g_{31} + b_{31} \text { (from eq.(2.8)) ... eq.(2.10)} \end{align}$$
Let us now find the left term of eq. (2.1) ${\partial E \over \partial u_{10}}$. As,
$E = {(t - r_{fc})^2\over 2}$, where t is given(expected) output in the training data,
$$\begin{align} {\partial E \over \partial u_{10}}&={\partial {(t - r_{fc})^2\over 2}\over \partial u_{10}}\\ &={1 \over 2}.{\partial {(t - r_{fc})^2}\over \partial u_{10}} \text{ ... eq.(2.11)}\\ \end{align}$$
For a partial derivative, the chain rule is:
$${\partial x \over \partial y}={\partial x\over \partial z}.{\partial z\over \partial y}$$
Applying the chain rule to right term of eq.(2.11):
$$\text{Here, } z = {t - r_{fc}},x=z^2, \text{ and } y=u_{10} \text{ ,therefore, }\\ \begin{align} {1 \over 2}.{\partial {(t - r_{fc})^2}\over \partial u_{10}}&={1 \over 2}.{\partial x\over \partial y}\\ &={1 \over 2}.{\partial x\over \partial z}.{\partial z\over \partial y}\\ &={1 \over 2}.{\partial z^2\over \partial z}.{\partial z\over \partial y}\\ &={1 \over 2}.{\partial (t - r_{fc})^2\over \partial (t - r_{fc})}.{\partial (t - r_{fc})\over \partial u_{10}}\\ &={1 \over 2}.{2(t - r_{fc})}.{\partial (t - r_{fc})\over \partial u_{10}} \text{ ... eq.(2.12)}\\ \end{align}$$
Further,
$$\begin{align} {\partial (t - r_{fc})\over \partial u_{10}}&={\partial (t - w_{fc}.r_p)\over \partial u_{10}}\\ &={\partial t\over \partial u_{10}} - {\partial w_{fc}.r_p\over \partial u_{10}}\\ \text { from eq.(1.4) we know } r_p = u_{10} \text{,therefore,}\\ {\partial (t - r_{fc})\over \partial u_{10}}&={\partial t\over \partial u_{10}} - w_{fc}.{\partial u_{10}\over \partial u_{10}}\\ &=0 - w_{fc} \text{ (as,} {\partial C \over \partial x}=0 \text { and } {\partial x \over \partial x}=1, \text {where C is a constant)},\\ \end{align}$$
Applying value of ${\partial (t - r_{fc})\over \partial u_{10}}$ in eq(2.12),
$$\begin{align} {1 \over 2}.{\partial {(t - r_{fc})^2}\over \partial u_{10}}&={1 \over 2}.{2(t - r_{fc})}.{\partial (t - r_{fc})\over \partial u_{10}}\\ &={1 \over 2}.{2(t - r_{fc})}.(0 - w_{fc})\\ &=(t - r_{fc}).(0 - w_{fc})\\ \end{align}$$
Applying value of ${1 \over 2}.{\partial {(t - r_{fc})^2}\over \partial u_{11}}$ in eq.(2.11),
$$\begin{align} {\partial E \over \partial u_{10}}&={\partial {(t - r_{fc})^2\over 2}\over \partial u_{10}}\\ &=(t - r_{fc}).(0 - w_{fc})\\ &=(t - r_{fc}).(-w_{fc}) \text{ ... eq.(2.13)}\\ \end{align}$$

Applying values from eq.(2.8) and eq.(2.13) in eq. (2.1),
$$\begin{align} {\partial E \over \partial w_{00}}&={\partial E \over \partial u_{10}}.{\partial u_{10} \over \partial w_{00}} \\ &={\partial E \over \partial u_{10}}.(i_{31}) \text { (from eq.(2.10))} \\ &={(t - r_{fc}).(-w_{fc})}.(r_{31} + g_{31} + b_{31}) \text { (from eq.(2.5)) ... eq. (2.14)} \\ \end{align}$$

Eq.(2.14) can also be converted to a general equation that covers both cases 1 and 2. This general equation is also useful if we use a different activation function other than ReLu. This general form of eq. (2.14) is:
$$\begin{align} {\partial E \over \partial w_{00}}&={\partial E \over \partial u_{10}}.{\partial u_{10} \over \partial w_{00}} \\ &={(t - r_{fc}).(-w_{fc})}.{\partial u_{10} \over \partial w_{00}}\\ &={(t - r_{fc}).(-w_{fc})}.{\partial f(c_{10}) \over \partial w_{00}} \text { ... eq.(2.15)}\\ \end{align}$$
Here, $f(x)$ is the activation function.

Using similar steps as for ${\partial E \over \partial w_{00}}$ let us now compute ${\partial E \over \partial w_{10}}$,

Applying the chain rule as before,
$$\begin{align} {\partial x \over \partial y}&={\partial x\over \partial z}.{\partial z\over \partial y}\\ \text{Here, } z = u_{10}, x&=E, \text{ and } y=w_{10} \text{ ,therefore, }\\ {\partial E \over \partial w_{10}}&={\partial E \over \partial u_{10}}.{\partial u_{10} \over \partial w_{10}} \text{ ... eq.(3.1)}\\ \end{align}$$
Solve for the right term ${\partial u_{10} \over \partial w_{10}}$ in eq.(3.1):
$$\begin{align} {\partial u_{10} \over \partial w_{10}}&={\partial (max(0,c_{10})) \over \partial w_{10}} \text{ ... eq.(3.2)} \end{align}$$
Applying the chain rule to eq.(3.2),
$$\begin{align} {\partial x \over \partial y}&={\partial x\over \partial z}.{\partial z\over \partial y}\\ \text{Here, } z = c_{10}, x&=max(0,c_{10}), \text{ and } y=w_{10} \text{ ,therefore, }\\ {\partial (max(0,c_{10})) \over \partial w_{10}}&={\partial max(0,c_{10})\over \partial c_{10}}.{\partial c_{10}\over \partial w_{10}} \text{ ... eq.(3.3)}\\ \end{align}$$
We already know,
$${d {{max(0,x)} \over {dx}}}= \begin{cases} 1, & \text {if x > 0}\\ 0, & \text {if x < 0}\\ undetermined, & \text {if x = 0 ... eq.(3.4)} \end{cases}$$

Case 1: $x \lt 0$,

In eq.(3.3),
$$\begin{align} {\partial (max(0,c_{10})) \over \partial w_{01}}&={\partial max(0,c_{10})\over \partial c_{10}}.{\partial c_{10}\over \partial w_{10}}\\ &={\partial 0\over \partial c_{10}}.{\partial c_{10}\over \partial w_{10}} \text { (applying value of } max(0,c_{10}) \text { from eq.(3.4))}\\ &=0.{\partial c_{10}\over \partial w_{10}}\\ &=0\text{ ... eq.(3.5)} \end{align}$$
Putting value from eq.(3.5) in eq.(3.2),
$$\begin{align} {\partial u_{10} \over \partial w_{10}}&={\partial (max(0,c_{10})) \over \partial w_{10}}\\ &=0 \end{align}$$
Putting value of $\partial u_{10} \over \partial w_{10}$ in eq.(3.1),
$$\begin{align} {\partial E \over \partial w_{10}}&={\partial E \over \partial u_{10}}.{\partial u_{10} \over \partial w_{10}}\\ &={\partial E \over \partial u_{10}}.0\\ &=0\\ \end{align}$$
Case 2: $x \gt 0$,
In eq.(3.3),
$$\begin{align} {\partial (max(0,c_{10})) \over \partial w_{10}}&={\partial max(0,c_{10})\over \partial c_{10}}.{\partial c_{10}\over \partial w_{10}}\\ &={\partial c_{10}\over \partial c_{10}}.{\partial c_{10}\over \partial w_{10}} \text { (applying value of } max(0,c_{10})=c_{10} \text { from eq.(3.3))}\\ &=1.{\partial c_{10}\over \partial w_{10}}\\ &={\partial c_{10}\over \partial w_{10}} \text{ ... eq.(3.6)} \end{align}$$
Solving for eq.(3.6),
$$\begin{align} {\partial c_{10}\over \partial w_{10}}=&\partial ((r_{20}.w_{11} + r_{30}.w_{01} + r_{21}.w_{10} + r_{31}.w_{00}) + \\ &(g_{20}.w_{11} + g_{30}.w_{01} + g_{21}.w_{10} + g_{31}.w_{00}) + \\ &{(b_{20}.w_{11} + b_{30}.w_{01} + b_{21}.w_{10} + b_{31}.w_{00}))\over \partial w_{10}} \text { (substituting full value of } c_{10} \text{)}\\ =&{\partial {(r_{20}.w_{11})} \over \partial w_{10}}+ {\partial {(r_{30}.w_{01})} \over \partial w_{10}} + {\partial {(r_{21}.w_{10})} \over \partial w_{10}} + {\partial {(r_{31}.w_{00})} \over \partial w_{10}} +\\ &{\partial {(g_{20}.w_{11})} \over \partial w_{10}}+ {\partial {(g_{30}.w_{01})} \over \partial w_{10}} + {\partial {(g_{21}.w_{10})} \over \partial w_{10}} + {\partial {(g_{31}.w_{00})} \over \partial w_{10}} +\\ &{\partial {(b_{20}.w_{11})} \over \partial w_{10}}+ {\partial {(b_{30}.w_{01})} \over \partial w_{10}} + {\partial {(b_{21}.w_{10})} \over \partial w_{10}} + {\partial {(b_{31}.w_{00})} \over \partial w_{10}}\\ =&{\partial {(r_{20}.w_{11})} \over \partial w_{10}}+ {\partial {(r_{30}.w_{01})} \over \partial w_{10}} + r_{21}.{\partial {w_{10}} \over \partial w_{10}} + {\partial {(r_{31}.w_{00})} \over \partial w_{10}}+\\ &{\partial {(g_{20}.w_{11})} \over \partial w_{10}}+ {\partial {(g_{30}.w_{01})} \over \partial w_{10}} + g_{21}.{\partial {w_{10}} \over \partial w_{10}} + {\partial {(g_{31}.w_{00})} \over \partial w_{10}}+\\ &{\partial {(b_{20}.w_{11})} \over \partial w_{10}}+ {\partial {(b_{30}.w_{01})} \over \partial w_{10}} + b_{21}.{\partial {w_{10}} \over \partial w_{10}} + {\partial {(b_{31}.w_{00})} \over \partial w_{10}} \text{ ... eq.(2.7)}\\ \end{align}$$
Note that the terms $({r_{20}.w_{11}}), ({r_{30}.w_{01}}), ({r_{31}.w_{10}}),({g_{20}.w_{11}}), ({g_{30}.w_{01}}), ({g_{31}.w_{10}}),({b_{20}.w_{11}}), ({b_{30}.w_{01}})$ and $({b_{31}.w_{10}})$ will not change w.r.t $w_{00}$ hence, can be treated as a constant C
As ${\partial C \over \partial x} = 0$, in eq.(3.7),
$$\begin{align} \partial c_{10} \over \partial w_{10}&={\partial {(r_{20}.w_{11})} \over \partial w_{10}}+ {\partial {(r_{30}.w_{01})} \over \partial w_{10}} + r_{21}.{\partial {w_{10}} \over \partial w_{10}} + {\partial {(r_{31}.w_{00})} \over \partial w_{10}} + \\ &{\partial {(g_{20}.w_{11})} \over \partial w_{10}}+ {\partial {(g_{30}.w_{01})} \over \partial w_{10}} + g_{21}.{\partial {w_{10}} \over \partial w_{10}} + {\partial {(g_{31}.w_{00})} \over \partial w_{10}} + \\ &{\partial {(b_{20}.w_{11})} \over \partial w_{10}}+ {\partial {(b_{30}.w_{01})} \over \partial w_{10}} + b_{21}.{\partial {w_{10}} \over \partial w_{10}} + {\partial {(b_{31}.w_{00})} \over \partial w_{10}} \\ &= 0 + 0 + r_{21}.{\partial {w_{10}} \over \partial w_{10}} + 0 + 0 + 0 + g_{21}.{\partial {w_{10}} \over \partial w_{10}} + 0 + 0 + 0 + b_{21}.{\partial {w_{10}} \over \partial w_{10}} + 0\\ &= 0 + 0 + r_{21} + 0 + 0 + 0 + g_{21} + 0 + 0 + 0 + b_{21} + 0 (\text {as, }{\partial x \over \partial x}=0 \text {)}\\ &= r_{21} + g_{21} + b_{21} \text{ ... eq.(3.8)} \end{align}$$
Putting value from eq.(3.8) in eq.(3.2),
$$\begin{align} {\partial u_{10} \over \partial w_{10}}&={\partial (max(0,c_{10})) \over \partial w_{10}}\\ &={\partial c_{10} \over \partial w_{10}} \text { (from eq.(3.6))}\\ &=i_{21} \text { (from eq.(3.8)) ... eq.(3.10)} \end{align}$$
Let us now find the left term of eq. (3.1) ${\partial E \over \partial u_{10}}$. This has already been computed in eq.(2.13) and given as:
$$\begin{align} {\partial E \over \partial u_{10}}&=(t - r_{fc}).(-w_{fc})\\ \end{align}$$
Applying values from eq.(3.8) and eq.(2.13) in eq. (3.1),
$$\begin{align} {\partial E \over \partial w_{10}}&={\partial E \over \partial u_{10}}.{\partial u_{10} \over \partial w_{10}} \\ &={\partial E \over \partial u_{10}}.(i_{31}) \text { (from eq.(3.10))} \\ &={(t - r_{fc}).(-w_{fc})}.(r_{21} + g_{21} + b_{21}) \text { (from eq.(2.13)) ... eq. (3.11)} \\ \end{align}$$
Eq.(3.11) can also be converted to a general equation that covers both cases 1 and 2. This general equation is also useful if we use a different activation function other than ReLu. This general form of eq. (3.11 ) is:
$$\begin{align} {\partial E \over \partial w_{10}}&={\partial E \over \partial u_{10}}.{\partial u_{10} \over \partial w_{10}} \\ &={(t - r_{fc}).(-w_{fc})}.{\partial u_{10} \over \partial w_{10}}\\ &={(t - r_{fc}).(-w_{fc})}.{\partial f(c_{10}) \over \partial w_{10}} \text { ... eq.(3.12)}\\ \end{align}$$
Here, $f(x)$ is the activation function.

It will be clear by now that any ${\partial E \over \partial w_{xx}}$ is the term ${(t - r_{fc}).(-w_{fc})}$ multiplied by the element in the input matrix $w_{xx}$. Therefore, for the remaining weights $w_{01}$ and $w_{11}$:
$$\begin{align} {\partial E \over \partial w_{01}}&={(t - r_{fc}).(-w_{fc})}.(r_{30} + g_{30} + b_{30}) \text { ... eq.(4.1)}\\ {\partial E \over \partial w_{11}}&={(t - r_{fc}).(-w_{fc})}.(r_{20} + g_{20} + b_{20}) \text { ... eq.(5.1)}\\ \end{align}$$
and their general form is:
$$\begin{align} {\partial E \over \partial w_{01}}&={(t - r_{fc}).(-w_{fc})}.{\partial f(c_{10}) \over \partial w_{10}} \text { ... eq.(4.2)}\\ {\partial E \over \partial w_{11}}&={(t - r_{fc}).(-w_{fc})}.{\partial f(c_{10}) \over \partial w_{11}} \text { ... eq.(5.2)}\\ \end{align}$$
A further general form to represent eqs.(2.15),(3.12), (4.2) and (5.2) for any weight $w_{xy}$ is:
$${\partial E \over \partial w_{xy}}={(t - r_{fc}).(-w_{fc})}.{\partial f(c_{ij}) \over \partial w_{xy}} \text { ... eq.(6.1)}$$
where, $c_{ij}$ is the output from the pooling function and $w_{xy}$ is the filter weight.

Weight update rule:

Once the direction of the weight update is determined using ${\partial E \over \partial w_{xy}}$ for a weight $w_{xy}$, we update the weights based on the same rule as for a standard NN. Therefore,

If $\eta$ is the learning rate and controls the numeric value by which weights are changed:

The weights for FC layer ($w_{fc}$) layer will be updated as:
$$\begin{align} \Delta w_{fc}&=-\eta.{\partial E\over \partial w_{fc}}\\ &=-\eta.{(t - r_{fc}).(-r_p)} \text { (from eq.(1.3))} \\ &=\eta.{(t - r_{fc}).(r_p)}\\ \Rightarrow w_{fc} &= w_{fc} + \Delta w_{fc} \end{align}$$
The weights for filter ($w_{00},w_{10},w_{01},w_{11}$) will be updated as:
$$\begin{align} \Delta w_{00}&=-\eta.{\partial E\over \partial w_{00}}\\ &=-\eta.{(t - r_{fc}).(-w_{fc})}.({\partial f(c_{10}) \over \partial w_{00}})\text { (from eq.(2.15))} \\ &=\eta.{(t - r_{fc}).(w_{fc})}.({\partial f(c_{10}) \over \partial w_{00}})\\ \Rightarrow w_{00} &= w_{00} + \Delta w_{00} \end{align}$$
$$\begin{align} \Delta w_{10}&=-\eta.{\partial E\over \partial w_{10}}\\ &=-\eta.{(t - r_{fc}).(-w_{fc})}.({\partial f(c_{10}) \over \partial w_{10}})\text { (from eq.(3.6))} \\ &=\eta.{(t - r_{fc}).(w_{fc})}.({\partial f(c_{10}) \over \partial w_{10}})\\ \Rightarrow w_{10} &= w_{10} + \Delta w_{10} \end{align}$$
$$\begin{align} \Delta w_{01}&=-\eta.{\partial E\over \partial w_{01}}\\ &=-\eta.{(t - r_{fc}).(-w_{fc})}.({\partial f(c_{10}) \over \partial w_{01}})\text { (from eq.(4.2))} \\ &=\eta.{(t - r_{fc}).(w_{fc})}.({\partial f(c_{10}) \over \partial w_{01}})\\ \Rightarrow w_{01} &= w_{01} + \Delta w_{01} \end{align}$$
$$\begin{align} \Delta w_{11}&=-\eta.{\partial E\over \partial w_{11}}\\ &=-\eta.{(t - r_{fc}).(-w_{fc})}.({\partial f(c_{10}) \over \partial w_{11}})\text { (from eq.(5.2))} \\ &=\eta.{(t - r_{fc}).(w_{fc})}.({\partial f(c_{10}) \over \partial w_{11}})\\ \Rightarrow w_{11} &= w_{11} + \Delta w_{11} \end{align}$$

Note that if the output of the pooling layer is different than $c_{10}$, you have to change the input values ($i_{xy}$) used in the weight update rule.

This concludes the derivation of backpropagation for a CNN with 3 input matrices. In part-IV, we look how to derive backpropagation in presence of 2 sequential convolutional and ReLu layers. This is a step towards deriving backpropagation for a deep-learning (multi-layered) CNN.